Referring to FIGS. 2 and 3, the prior art and the problems thereof will be described. FIG. 2 shows an example of a so-called cascode circuit containing a composite semiconductor device, and FIG. 3 shows another example of a circuit containing a composite semiconductor device.
In FIG. 2, the reference numeral 1 designates a power source, 2 a load, 3 a bipolar transistor (hereinafter referred to as BPT), 4 a field effect transistor (hereinafter referred to as FET) of the MOS type or the like in this example, and 6 a Zener diode.
The channel between a collector C and an emitter E of the BPT 3, and the channel between a drain D and a source S of the FET 4, are connected in series with each other at the emitter E and the drain D, so as to switch a current IC (referred to as a collector current herein) supplied from the power source 1 to the load 2. The combination of the transistors 3 and 4 is referred to as a composite transistor herein. The Zener diode 6 is connected between a base B of the BPT 3 and the source S of the FET 4 so as to make the cathode side of the Zener diode 6 connected with the base B.
A switching signal with a voltage e.sub.G for determining whether to turn on/off the collector current IC is applied between a gate G and the source of the FET 4 through a driving circuit (not shown), and, when the FET 4 is to be turned on by rendering the channel conductive, a base current IB 1 is supplied to the base B through a base power source 8 provided between the base B of the BPT 3 and the source S of the FET 4.
This circuit is intended to obtain a composite switching element with high speed and a high withstanding voltage by combining a high speed and low withstanding voltage FET 4 and the low speed and high withstanding voltage bipolar transistor 3. The switching speed of an FET is generally higher than that of a bipolar transistor.
First, the manner of turning on the composite of the transistors 3 and 4 will be described. In this circuit, since the FET 4 is connected with the emitter E of the BPT 3 so as to enable the current IB 1 to be switched, it is possible to apply a relatively high base voltage e.sub.B to the base B of the BPT 3. Therefore, if a switching signal voltage e.sub.G for turning on the transistor 4 is applied to the FET 4, a voltage VDS between the drain D and the source S steeply falls to make the base current IB 1 steeply rise, thereby quickly turning on the BPT 3, so that both transistors 3 and 4 are on. In this case, the Zener diode 6 is in its off-state (non-conductive state).
When the composite transistors 3 and 4 are turned off, on the other hand, if a switching signal voltage e.sub.G for turning off the transistor 4 is applied to the FET 4, the voltage VDS across the drain-source of the FET 4 steeply rises, which cuts off the collector current IC flowing into the FET 4. At that moment, the collector current IC flowing through the base-emitter of the BPT 3 is commutated into the Zener diode 6. The charge carriers at the base portion of the BPT 3 are quickly depleted, and the transistor 3, and hence the composite transistors 3 and 4, are quickly turned off to cut the collector current IC.
The reason why the Zener diode 6 is used in the commutation path is so that the voltage VDS across the drain-source of the FET 4 can be prevented from exceeding a threshold voltage that prevents switching when the FET 4 is to be turned off. When the collector current IC is commutated in the manner described above, the base current IB 1 supplied from the base power source 8 into the base B of the BPT 3 can be prevented from uselessly shunting into the Zener diode 6, and is instead used more efficiently as the base current when the composite transistors 3 and 4 are in the on-state. In the circuit shown in FIG. 2, however, there is concern because the base power source 8 must be separately provided to drive the base of the BPT 3.
FIG. 3 shows a circuit for solving the problem described above for FIG. 2. In the drawing, the reference numeral 5 designates an auxiliary FET which is driven by the same switching signal with voltage e.sub.G as that of FET 4, and which supplies a base current from a power source (the collector C of BPT 3) to the base of BPT 3 when the auxiliary FET is turned on. In the circuit, although the main circuit current can be switched by a single control signal (a switching signal with voltage e.sub.G), there are problems because the saturation voltage of the BPT 3 is high due to the operating resistance of the auxiliary FET 5, so the resulting power loss is large. Furthermore, the auxiliary FET 5 is expensive because of its high withstanding voltage characteristics.